Water is a natural byproduct of the combustion of hydrocarbon or fossil fuels. Permits for water are becoming increasingly difficult to obtain for power plants, which consume relatively large volumes of water during operation. In some cases, the difficulty with obtaining water permits for wells or use of surface water may preclude construction of a needed power plant. Thus, recovering water from power plants is desirable to obviate the need of obtaining water permits.
Fossil fuel exhaust or flue gas, such as that exhausted from a combustion turbine engine or downstream of a coal-fired boiler, can contain 10% volume concentration of water depending on ambient conditions, fuel composition, inlet air treatment, fuel treatment, flue gas treatment and other factors. At this concentration, water has a partial pressure of 1.47 psia and a saturation temperature of 115° F. If the flue gas were cooled and the vapor pressure lowered, a portion of that water could be recovered. It is known that cooling the exhaust stream in a condenser to below the precipitation temperature of the moisture in the exhaust gas will result in the condensation of a portion of the moisture contained in the exhaust gas. The quantity and percentage of the moisture recovered is a function of the temperature to which the exhaust can be cooled by the condenser.
However, since the recovery of the water is expensive, the usual site for such plants would be a desert where daytime temperatures are high, frequently approaching 118° F. Ambient air is commonly the ultimate heat sink for condensers, and the ambient air temperature thus determines the amount of moisture that can be removed by a condenser. In an arid desert environment, where it is most likely that water recovery will be desired the effectiveness of water removal by an ambient air-cooled condenser is limited. Given such high ambient temperatures and the limits of heat exchange equipment, direct condensation alone becomes technically untenable.